Have a couple of shunts with 5/16" studs laying around. Have a number of high load circuits I need to energize, the highest draw one is the windlass. They have their own circuit breakers or fuses in the circuit and are switched outside the electrical panel. Is there any reason I can't connect a heavy gauge wire to the battery switch and use the shunts as a junction to connect the leads ??? Won't be using the shunt for an amp gauge. Only one circuit will be drawing at a time. The DC power panel for the boat is separately energized from the battery switch.

Electrically this will work, as long as you don't mind adding a few milliohms to the circuit. And as long as the shunt rating is truly higher than you can ever have running through it.

But why don't you get some actual power lugs or high-current terminal strip? They aren't expensive and they aren't going to confuse anybody. If I had to troubleshoot your boat and saw the shunts wired as you propose I might be a bit puzzled.

Isn't the normal operational use of a shunt just this? All the boat current goes through it - the only difference is that there is usually only one connection on each end. Most shunts I have seen have rather short bolts, so making several connections on one could be difficult.